1. Field of the Invention
The present invention relates to a process and equipment for treating an ammonium containing liquid, in particular, to a technology for biologically denitrifying ammonium in an ammonium containing liquid generated in various fields such as the field of wastewater treatment and the field of fine chemicals.
2. Description of the Related Art
Since the Mikawashima sewage treatment plant in Tokyo began operation of full-scale sewage treatment in 1922, sewage treatment plants have operated not only treatment of organic substances but also treatment of nitrogen. In particular, governments of major cities have invested in sewage treatment so that the sewerage coverage finally reached more than 90%.
However, the sewerage has allowed almost no closed water areas to meet environmental standards. This is largely because of abnormal propagation of algae as an internal factor in water areas, but also because of inflow wastewater as an external factor still to be eliminated. In particular, nitrogen is largely involved in the external factor.
Thus, a large amount is nitrogen is contained in sewage or wastewater in the form of ammonium nitrogen. Ammonium containing liquids with low to high concentrations are generated in industrial wastewater, photographic development wastewater, and wastewater from chemical factories to produce chemical products, for example. These ammonium containing liquids, if discharged as wastewater, cause eutrophication and lowering the level of dissolved oxygen in waters, and thus it is necessary to purge ammonium before discharging the liquids. Further, it is necessary to purge ammonium from liquid chemical products containing ammonium in order to increase purity of the chemical products.
In these circumstances, ammonium containing liquids with low concentrations have been conventionally oxidized by chlorine or biologically treated. In the chlorine treatment, chlorine is reacted with ammonium to remove ammonium and produce chloramine at the same time. The chlorine treatment is not used for treating ammonium containing liquids with middle to high concentrations, since the chloramine is highly bactericidal and may disturb the ecosystem in the environment, and the treatment requires a large amount of chlorine. Such liquids are typically biologically treated by nitrification and denitrification.
The biological treatment by nitrification and denitrification is performed in sewage treatment plants and wastewater treatment plants by nitrification in which ammonium is converted into nitrate through nitrite by nitrifying bacteria, and denitrification in which nitrate is converted into nitrogen gas by denitrifying bacteria. However, the treatment of ammonium containing liquids by nitrification and denitrification requires an organic substance in denitrification reaction, and requires methanol to be added in an amount three times greater than nitrogen as the organic substance. In order to remove nitrogen in a stable manner, such treatment must be operated at a low nitrogen load of 0.2 to 0.4 kg-N/m3/day. Accordingly, as the concentration of ammonia is higher, a greater amount of expensive methanol is used. Thus, a large tank is necessary as a treatment tank, and the treatment requires not only an initial cost but also a large running cost, disadvantageously.
In this situation, a process for treating wastewater utilizing anaerobic ammonium oxidation has been attracted attention from a long time ago (for example, Japanese Patent Application Laid-Open No. 2001-037467). The anaerobic ammonium oxidation is a method comprising converting a part of ammonium to nitrite by nitrite-type nitrification reaction using ammonium oxidizing bacteria, with ammonium as an electron donor and nitrite as an electron acceptor, and simultaneously denitrifying the nitrite and the remaining ammonium using anaerobic ammonium oxidizing bacteria without requiring an organic substance. The anaerobic ammonium oxidation requires only a small amount of oxygen in nitrification reaction, does not require an organic substance (such as methanol) in denitrification reaction, and thus can be operated at a considerably reduced running cost, advantageously. The anaerobic ammonium oxidation can reduce the amount of sludge generated, for example, advantageously, and is assumed to be an effective wastewater treatment method in the future.
A large number of processes for treating ammonium containing liquids as wastewater by anaerobic ammonium oxidation disclosed in Japanese Patent Application Laid-Open No. 2001-37467 have been proposed which are allegedly advantageous in significantly reducing the running cost. However, these processes have difficulty in practical use and have not been commonly employed.
This is because, in anaerobic ammonium oxidation, the reaction involves nitrite as an electron acceptor but does not involve nitrate as an electron acceptor, and thus nitrite must be supplied in a stable manner.
Specifically, when wastewater has a low ammonium nitrogen concentration, ammonium is easily oxidized to nitrate, and the amount of ammonium oxidized to nitrite is necessarily small. Thus, the wastewater has a ratio of ammonium to nitrite differing from such a ratio suitable for anaerobic ammonium oxidizing bacteria. In this manner, nitrification reaction of converting a part of ammonium to nitrite is not stable, and thus the concentration of nitrite reacted with ammonium is easily changed over time. Since it is thus difficult to react nitrite with ammonium by denitrification constantly at a preferable ratio between them, ammonium containing liquids cannot be treated in a stable manner, quality of the treated liquids tends to vary, and anaerobic ammonium oxidation treatment cannot be efficiently carried out. In particular, wastewater such as sewage tends to have a low ammonium nitrogen concentration, and it is thus difficult to treat such wastewater stably for a long time.
The present invention has been achieved in view of such circumstances. An object of the present invention is to provide a process and equipment for treating an ammonium containing liquid which can perform high-speed denitrification of an ammonium containing liquid by anaerobic ammonium oxidation or the like, without being affected by the change in concentrations of nitrogen components in the ammonium containing liquid to be treated, and can provide a treated liquid with high quality constantly stably.